The present invention relates to the regulation of the frequency or frequencies of oscillators, which may find application in a circuit for maintaining the frequency of the local oscillator used in color signal demodulation within a predetermined range, so that it can be locked to the incoming color burst signal.
At present, many color decoders used in multi-standard color television receivers use a number of crystals to generate reference frequencies for the different color systems. Since both the crystals and the associated electric circuitry have tolerances, correct locking of the local color subcarrier to the incoming color burst can only be assured if these frequencies are controlled to be within a relatively small frequency range around a nominal value. This value may be different for different color systems.
A system for changing the nominal frequency without switching to a different crystal is shown in U.S. Pat. No. 4,682,209 (Nillesen). In the system described in that patent, the frequency of a digital oscillator is controlled by insertion of a quiescent frequency number to which is added a frequency error corresponding to the deviation from the desired quiescent frequency. However, there are no devices whose function it is to maintain the local oscillator frequency within a given frequency range.
U.S. Pat. No. 4,229,759 describes a control loop for a voltage controlled oscillator which may be the local oscillator in a color TV receiver. The burst signal and local oscillator signals are both applied to a wideband analog multiplier circuit whose outputs are sampled during the burst interval by sample-and-hold circuits. The outputs of the sample-and-hold circuits constitute the control signal for the local oscillator. The sample-and-hold circuit includes capacitors which are symmetrically bidirectionally charged and discharged to accommodate positive and negative frequency (phase) deviations. There is no monitoring in this system to return the local oscillator frequency to within a predetermined frequency range so that it can be locked to the burst.
In EP-A 0,132,196 the color subcarrier frequencies required for the different color television systems are generated by a voltage controlled oscillator whose output frequency is corrected on the basis of comparison with two reference frequencies.